The present invention relates to an improvement in and relating to a heating or cooling system based on thermally coupled intermittent absorption heat cycles, which utilizes the heat generation and absorption incident to a reversible absorption and desorption of a working medium to an absorbent material.
A majority of the energy consumed by the human being is consumed as heat energy and, at the same time, most of it is wasted. On the other hand, heat produced as a result of the combustion of chemical fuels is at an excessively high temperature for heating purpose. Accordingly, where heat of a relatively low temperature is demanded, it is desirable to increase the heat value by utilizing a high temperature heat source and absorbing heat from the atmospheric air. Conversely, it is often desired to raise the temperature of waste heats of low temperature or to realize a lower temperature than the temperature of a heat sink such as a room temperature by the utilization of a source of thermal energy.
Absorption heat cycles are known as an effective means for the utilization of thermal energies for these purposes. Of them, the absorption heat pump for heating and cooling is currently being watched with keen interest because of the structural simplicity and also of the minimized requirement for auxiliary power. As a working medium used in the absorption heat pump, it can be contemplated to use such materials as a hydrogenated metal, an inorganic hydrate, an organic substance and zeolite. Working gaseous mediums include hydrogen, water vapour and ammonium.
The intermittently driven heat pump devices hitherto well known, while considered to have numerous advantages such as energy-saving, low-noise and low-vibration features, have not yet answered the purpose of practical utility as much as the compression type and the absorption type. The reason therefor is that performances of these heat pump devices including the coefficient of performance (COP) are relatively low and the conditions in which they are utilized are limited, and they also have a high manufacturing cost. Furthermore, it is difficult to solve the problem from the technical viewpoint. Accordingly, an important problem with the intermittently driven heat pump devices is to increase the performance and to reduce the price thereof. In addition, it is often desired to lower the temperature attained in a cooling cycle for creating a lower temperature than that of a heat sink, and to raise the temperature attained in a temperature-upgrading heat pump designed to obtain a higher temperature than that of a heat source.
Hereinafter, the structure of the prior art heat pump device and the problem inherent therein will be described by way of an example wherein a hydrogenated metal is used.
The temperature versus pressure relationship exhibited by the usual prior art intermittent heat pump cycle is shown in FIG. 1.
While two kinds of hydrogenated metals capable of exhibiting different temperature versus equilibrium pressure characteristics are used, the temperature of one of the hydrogenated metals exhibiting a lower equilibrium pressure for a given identical temperature, which compound is hereinafter referred to as MH1, is raised to a temperature TH by inputing heat from a high temperature heat source. This condition is shown as Phase A. In this case, MH1 increases in pressure with increase of the temperature and hydrogen of MH1 can move, by the effect of the difference in pressure, to the other hydrogenated metal (referred to as MH2) exhibiting a higher equilibrium pressure and, therefore, the temperature attained by MH2 is TM, this condition being shown as a Phase B. When MH1 maintained at the temperature TM is subsequently communicated with MH2, the hydrogen of MH2 moves to MH1. As a result of this movement, the temperature of MH2 decreases to a value TL below the ambient temperature by the effect of endothermic reaction (Phase C) while MH1 is heated (Phase D). By repeating the process consisting of the sequence of Phases A to D, it is possible to realize a heat pump cycle wherein heat of the temperature TM can be produced by the reaction taking place at Phases B and D while heat from the high temperature heat source of the temperature TH is inputed at Phase A, and a heat pump cycle for refrigeration wherein the cooling effect of the temperature TL can be produced by the endothermic reaction taking place at Phase C.
In the heat pump cycle shown in FIG. 1, although it is watched with keen interest as an energy saving device because the coefficient of performance (COP) is greater than 1.0 and the thermal output is greater than the thermal input, the COP for cooling is about 0.4 to 0.5 and, therefore, the performance thereof is not favorable relatively.
In this heat pump cycle now under discussion, when the temperature TH of the input heat source is selected to be of a high quality, relatively high temperature, for example, 200.degree. to 300.degree. C., the COP decreases rather than increasing and, therefore, the high quality, high temperature heat source can not be effectively and efficiently utilized.
Hereinafter, the prior art heat pump cycle for heat upgrading purpose will be described by way of an example wherein hydrogenated metals are employed.
The usual prior art heat pump cycle for heat upgrading purpose exhibits a temperature versus equilibrium pressure relationship as shown in FIG. 2. Two kinds of the hydrogenated metals capable of exhibiting different temperature versus equilibrium pressure characteristics are utilized, and when the hydrogenated metal MH1 of lower equilibrium pressure, which has sufficiently absorbed hydrogen is heated to a temperature TM (Phase E), is communicated with the dehydrogenated metal MH2 of higher equilibrium pressure at the identical temperature which has sufficiently desorbed hydrogen of a temperature TL, the hydrogen absorbed by MH1 moves to MH2 (Phase F). At this time, although MH2 produces heat by the effect of the exothermic reaction, the heat so produced is discharged to the atmosphere.
When MH2 is heated to the temperature TM and is then communicated with MH1 having desorbed the hydrogen, the hydrogen moves from MH2 to MH1. At this time, MH1 is heated by the exothermic reaction with its temperature increasing from the value TM to a value TH and produces heat (Phase H) at an equilibrium temperature for a pressure corresponding to the equilibrium pressure attained by MH2 at the temperature TM. By repeating the process consisting of the sequence of Phases E to H, it is possible to produce heat of a higher temperature TH than the temperature TM of the input heat source.
Although the heat pump cycle for heat upgrading purpose has a relatively high utility in that heat of a higher temperature than that of the input heat source can be produced, demands have been made to further maximize the temperature attained by the heat pump cycle. However, given the temperature of the input heat source and the temperature of heat discharged on the low-temperature side, the range over which the temperature can be increased is automatically fixed. Similarly, even in the case of the heat pump cycle for cooling purpose, demands have been made to minimize the temperature attained by the heat pump cycle. To meet these demands, it is necessary to increase the heating temperature to suit to the situation resulting in the problem that the high pressure side tends to be highly pressurized.
An idea of a multi-stage cycle has been introduced in an attempt to meet the above described demands.
FIG. 3 illustrates the temperature versus equilibrium pressure exhibited by the double-effect intermittent cycle in which the high temperature heat source can be effectively utilized. In the graph of FIG. 3, A, B, C and D correspond respectively to A, B, C and D shown in FIG. 1. TH represents the heating temperature of the first stage cycle. If another cycle performing the process consisting of the sequence of A', B', C and D using the same absorbent material is formed with the temperature at B' being somewhat higher than the temperature at A and if the heat produced at B' is transported to A by the use of any heat transporting means to make it available for heating MH1 of the cycle of A, B, C and D, this cycle will be driven by waste heat produced by the cycle of A', B', C and D and, as a consequence, for the heat input at A' of the cycle of A', B', C and D, the low temperature heat output at C can be obtained from the two cycles and, therefore, the output increases as compared with that of a single-stage cycle with the result of an increased COP. Similarly, when it is considered as a heat pump cycle, since heat generation takes place at B and D of the cycle of A, B, C and D and also at D of the cycle of A', B', C and D, the output can increase and the coefficient of performance can also increase. In such case, the heating temperature increases from the value TH to a value TH' and, although it is a method applicable where the high heating temperature can be available, it may be said that the method effectively utilizes the high temperature of the heat source. In FIG. 3, the bold arrow-headed line represents the transportation of the heat and, for the purpose of simplification of the drawing, points A and B' are shown as having the identical temperature TH.
FIG. 4 illustrates the example wherein the cycles are two-staged for producing a lower temperature, wherein A, B, C and D correspond exactly to A, B, C and D of FIG. 1, respectively. In this connection, if another cycle of A, B, C' and D' is formed by the use of the same absorbent materials MH1 and MH2 so that the temperature at D' can be somewhat higher than that at C, and the heat generated at D' is removed by absorption at C of the cycle of A, B, C and D (the bold arrow-headed line representing the transportation of the heat), heat absorbent of a temperature TL' can be produced at C' of the cycle of A, B, C' and D' and, thus, the temperature TL' lower than the temperature TL can be obtained, it being noted that the temperature difference of TM-TL' is approximately twice the temperature difference of TM-TL.
FIG. 5 illustrates the example of the heat upgrading model, wherein E, F, G and H correspond respectively to E, F, G and H of FIG. 2. In this connection, if another cycle of E, F, G' and H' is formed by the use of the same absorbent materials MH1 and MH2 so that the point G' can be heated at a temperature slightly lower than the temperature TH of heat produced at H, the cycle of E, F, G' and H' can be driven by the heat produced at H of the cycle E, F, G and H with the result that heat of a temperature TH' is produced at H'. Thus, the temperature TH' higher than the temperature TH can be obtained, the temperature difference of TH'-TM being approximately twice that of TH-TM.
These methods are advantageous in that the high performance, the great coefficient of performance and the low or high temperature, which have not been achieved by the single-stage cycle, can be obtained. However, they have a disadvantage in that, as compared with the basic cycle of A, B, C and D (E, F, G and H), the difference between high and low pressures tends to increase in the cycle of A', B', C and D (E, F, G' and H'). Considering that the ordinates represents the logarithm of the pressure, the increase of the pressure difference reaches a considerably great value. Specifically, if the low pressure side is assumed to be about 2 atms. because a lower pressure head can not be selected in view of the pressure loss occurring during the flow of gas, the high pressure side in the usual cycle is about 8 atms. whereas the high pressure side in the cycle of A', B', C and D is higher than 25 atms., although it depends on the temperature range applied to the absorbent material. This poses a problem of safety and, in order to secure the safety, it is necessary to increase the pressure resistance of vessels for accommodating the absorbent materials by giving them a great wall thickness.
The coefficient of performance of the intermittent heat cycle is expressed by a ratio between the output, which is the balance between the heat of absorption or desorption by the absorbent material and the loss of sensible heat attributable to the heat capacity of both the absorbent material and the absorbent vessel, and the input which is the sum of said heat of absorption or desorption plus the loss of said sensible heat. Therefore, the higher the pressure resistance of the absorbent vessel, the higher the heat capacity thereof, resulting in a decreased coefficient of performance. Moreover, increase of the pressure range is invited by the increase of the temperature range applied to the absorbent material and, therefore, lowers the loss of sensible heat. Accordingly, unless care is taken, the output will become zero.
Furthermore, where the two-stage model is developed with a view to increasing the coefficient of performance of the first mentioned cooling cycle, the coefficient of performance COP2C thereof will be expressed by the following equation: EQU COP2C=COP1C1(1+COP1C2) (1)
wherein COP1C1 and COP1C2 represent the coefficient of performance of the cycle of A', B', C and D and that of A, B, C and D, respectively.
Accordingly, in the event that the coefficient of performance COP1C1 of the high pressure cycle becomes zero, the coefficient of performance COP2C becomes zero correspondingly.
The coefficient of performance COP2H exhibited when the cycle is used to produce heat is given by the sum of one and COP2C, namely: EQU COP2H=COP2C+1 (2)
The composite coefficient of performance COP2D of the two-stage model developed for the purpose of giving a lower temperature is expressed by the following equation: EQU COP2D=(COP1D1.times.COP1D2)/(1+COP1D2) (3)
wherein COP1D1 and COP1D2 represent the coefficient of performance of the cycle of A, B, C and D and that of the cycle A, B, C' and D', respectively.
The coefficient of performance COP2T of the two-stage model of the heat upgrading cycle developed to give a higher temperature is expressed by the following equation: EQU COP2T=(COP1T1.times.COP1T2)/(1+COP1T1.times.COP1T2-COP1T2) (4)
wherein COP1T1 and COP1T2 represent the coefficient of performance of the cycle of H, G, E and F and that of H', G', E and F, respectively.
Since both of COP1D1 and COP1D2 are of a value smaller than 1, COP2D takes an extremely small value. In addition, since both of COP1T1 and COP1T2 is of a value smaller than 0.5, COP2T takes an extremely small value.
As hereinbefore discussed, although the two-stage arrangement using the two single-stage intermittent heat cycles makes it possible to increase the coefficient of performance, and also to produce heat of high or low temperature, it is susceptible to an increase in pressure difference with the coefficient of performance reduced consequently. Moreover, even a multi-stage arrangement constructed by combining three or more cycles is equally susceptible to the increase in pressure difference and, accordingly, it necessarily results in the reduction in coefficient of performance.